Lubricants are generally mixtures of several components. The largest fraction of the blended lubricant is a mineral oil or synthetic basestock that typically makes up more than 80 percent of the total volume. The remainder of the lubricant consists of various additives which impart performance improving attributes such as antioxidancy, antiwear, foam reduction and the like. Additional additives, known as viscosity modifiers, are also sometimes added to thicken the lubricant and improve the viscosity versus temperature attributes of the lubricant. Viscosity modifiers are made of relatively high molecular weight polymeric molecules that can be quite viscous. Basestocks are much lower in viscosity. Consequently, lubricant blending equipment and methods usually necessitate dispensing components that span a wide viscosity range.
In order to make a blend in a laboratory, one typically transfers liquid lubricant components into the blending vessel by using pipettes. Standard pipettes are operated by air-displacement, i.e., controlling gas pressure inside the pipette. Vacuum is applied to pull liquid into the pipette and pressure is applied to expel liquid from the pipette. In many cases, use of a calibrated pipette results in accurate blending. However, pipetting and transferring high viscosity liquids, such as viscosity modifiers, may result in inaccuracies from several sources. The use of air or gas pressure to expel liquid from the pipette may result in different amounts of liquid transfer depending on the gas pressure and viscosity of the liquid. Viscous liquids generate significant resistance to the applied gas pressure and gas compression may result in less liquid than desired being ejected from the pipette. In addition, polymeric viscosity modifiers may form stringy residue near the tip inside the pipette, resulting in less liquid dispensed in the receiving vessel. These problems are especially severe when trying to make small laboratory blends which require a high degree of accuracy because small blends may only contain milligrams of total mass. Accurate blending requires that individual component volumes be measured with microliter accuracy, and component mass be measured with milligram or better accuracy.
A further limitation of air displacement pipettes is that they require connection to a pump or vacuum system. In the case of manually operated pipettes, a rubber bulb is typically utilized. However, in a robotic liquid handling system, tubing is typically connected to each pipette. In many cases, a system liquid is also used to help the transfer of the pump action to the pipette tips and an air gap is used to separate the system liquid from the liquid to be transferred (a combination of air and liquid displacement). This can be quite cumbersome when many pipettes are used. For example, if many blend components are being used, each component requires its own pipette to avoid having to continuously clean pipettes. With air displacement or combination of air/liquid displacement pipettes, each pipette must be connected to a pump, which may not be practical. Alternatively, one pipette may be utilized, but this necessitates repeated cleaning of the pipette between each use of a different component. In the case where a system liquid is used, there is also a possibility of cross-contamination between the system liquid and the lubricant additives.
High viscosity lubricant components are often derived from high molecular weight polymers. Thus, high viscosity lubricant components may degrade when subjected to high shear conditions. High shear results when a high viscosity lubricant is forced through a small orifice at high pressure, which may cause permanent rupture of molecular bonds. It is therefore desirable when pipetting high viscosity lubricant components to maintain a relatively low shear rate when ingesting them into the pipette, and also when expelling them from the pipette. In some cases, the blending process may be improved by heating high viscosity components thereby reducing their viscosity. It is desirable to minimize the need for heating components because lubricant components may degrade at elevated temperature.
A need exists for an improved method of accurately blending highly viscous additives into lubricants to alleviate the aforementioned issues associated with the prior art techniques of blending lubricants.